The present invention is directed generally to optical transmission systems. More particularly, the invention relates to optical switching devices, such as optical cross-connect switches, routers, add/drop multiplexers, and equalizers for use in optical systems.
Digital technology has provided electronic access to vast amounts of information. The increased access has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems to link the processing equipment.
In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal formats analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals, such as voice and video signals, into a higher bit rate signal that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal is separated from the higher bit rate signal following transmission through the optical system.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, i.e., far-UV to far-infrared. The multiple information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems are presently deployed as in point-to-point WDM serial optical links (“PTP-WDM”) interconnected by electrical signal regeneration and switching equipment. At the electrical interconnection sites in the PTP-WDM systems, all optical signals are converted to electrical signals for processing. Electrical signals can be dropped and/or added at the site or can be regenerated and retransmitted on nominally the same or a different wavelength along the same fiber path or switched to a different fiber path.
As would be expected, it can become extremely expensive to perform optical to electrical to optical conversions in PTP-WDM systems merely to pass signals along to the transmission path. The cost of electrical regeneration/switching in WDM systems will only continue to grow with WDM systems having increasing numbers of channels and transmission paths in the system. As such, there is a desire to eliminate unnecessary, and costly, electrical regeneration and switching of information being transported in optical systems.
Current optical systems already benefit from the use of limited capability, optical switching devices. For example, optical add/drop multiplexers provide optical access to a single transmission fiber to remove selected channels from a WDM signal. Optical add/drop multiplexers eliminate the need to convert all of the optical signals in the transmission fiber to electrical signals, when access to only a portion of the traffic being transmitted as optical signals is required.
As optical system capacity requirements continue to grow with demand, it will become increasingly necessary for optical systems to evolve from point to point optical link toward multidimensional optical networks. Numerous optical switching devices have been proposed as alternatives to electrical switching to enable multidimensional all optical networks. For example, U.S. Pat. Nos. 4,821,255, 5,446,809, 5,627,925 disclose various optical switch devices.
A difficulty with many proposed optical cross-connect switches is that the switches become overly complex with increasing numbers of optical channels and input/output ports. The interconnection of multiple fiber paths, each carrying multiple channels, also becomes extremely difficult to manage effectively.
In addition, the interconnection of multiple fiber paths can introduce additional complications in the system. For example, signal channels being switched between different fiber paths can have different power levels that introduce channel to channel power variations in optical paths exiting the optical switching devices. The power variations in optical switching devices are particularly troublesome, because the variations can propagate through numerous optical paths and degrade the performance throughout a portion of the system.
In addition, optical switching devices can also impose significant optical losses on the signal channels passing through the devices. As the number of ports, i.e., size, and functionality of the optical switching devices increases, it generally becomes necessary to amplify the signal channels to overcome splitting losses and other losses in the optical switching device. However, optical amplification to overcome losses associated with optical switching devices can further introduce variations in the signal channels passing through an optical amplifier.
Typically, optical amplifiers automatically control the characteristics of signal channels passing through the optical amplifier using either Automatic Power Control (APC) or Automatic Gain Control (AGC) schemes. In APC schemes, the amplifier gain will be varied according to the APC scheme to maintain the total output of the optical signal exiting the amplifier within a constant power range. As a result, if the number of channels varies, the individual signal channel powers can vary in APC schemes. In AGC schemes, the amplifier gain is maintained within a constant gain range. Therefore, if the signal channel powers or the number of signal channels varies, then the total output power of the optical signal channel will vary as the overall gain of the amplifier is maintained within its constant gain range. As with APC schemes, the individual signal channel powers can vary in AGC schemes.
Traditional AGC and APC schemes are typically not effective in optical switching device configurations. The ineffectiveness is because signal channels are often combined from multiple input optical paths, which can have different signal powers. Thus, while the signal channels in any given input optical path may be controlled, variations in signal power between the input paths can cause signal channel power variations in one or more of the output optical paths.
Signal channel power variations also can be inherently produced as a result of the optical switching device design. The various processes performed in the devices, such as demultiplexing, splitting, switching, adding, dropping, coupling, and multiplexing of various signal channels can each introduce variations in the signal channel power levels.
For example, in many optical systems and component designs it is common to include symmetrically designed demultiplexers and multiplexers. The symmetrical demux/mux construction provides for streamlined manufacturing of the products and the potential for bi-directional use in bi-directional systems. However, symmetrical demux/mux configurations can produce signal channel power variations, if deployed with all optical switching devices.
Analogously, dissimilar demux/mux configurations are often deployed in various optical switching devices. For example, wavelength selective demultiplexers can be used with non-wavelength selective combiners, such as N:1 couplers. These configurations can also introduce signal channel power variations.
Depending upon the number of channels in the WDM system, one or more stages of non-wavelength selective splitting and combining can be used along with various filtering techniques applied to each wavelength. For example, see U.S. Pat. No. 5,446,809 (the “'809 patent”), which is incorporated herein by reference. Unfortunately, as the number of channels in WDM systems continues to increase, non-wavelength selective splitting and coupling can become impractical even when optical amplification is used to overcome the passive losses.
Another source of signal channel degradation in optical switching devices occurs when the switches incompletely block signal channels, thereby allowing leakage of unwanted signal channels through the device. Leakage of unwanted signal channels will degrade the signal quality of the signal channels being passed through the devices by creating cross-talk interference. Significant levels of cross-talk can destroy the information carried by signal channels being passed through the device. The amount of crosstalk that occurs through an on/off switch can be characterized by the extinction ratio of the switch, which is the ratio of power transmitted through the switch in the on state over the off state.
There are numerous types of optical switches, or gates, such as mechanical, thermo-, acousto-, and electro-optic, doped fiber and semiconductor gates, and tunable filters, that are available for use in optical switching devices. Thermo-, acousto-, and electro-optic switches are appealing, because of the relatively low cost and solid state characteristics. However, these switches often have extinction ratios only on order of ˜20 dB. The low extinction ratios can introduce unacceptable levels of cross-talk in optical systems. As such, these types of switches are typically limited to use in systems in which minor signal degradation can be tolerated or the degradation does not accumulate from multiple switches.
Conversely, mechanical line or mirror switches can have excellent extinction ratios, but the moving parts associated with mechanically moving and aligning the switch can pose a reliability problem over the long term. Doped fiber and semiconductor on/off gates can provide good extinction, but require active components to maintain the gate configurations, which increases cost and decreases reliability. Tunable filters can also provide good extinction ratio in a switch mode, but the filters may require active components that have maintainable, stable tuning characteristics.
The various signal degradation mechanisms that can be introduced in an optical system by prior art optical switching devices have been a contributing factor to the industry's inability to develop and deploy reliable optical switching devices to enable the creation of all-optical, or “transparent”, networks. Accordingly, there is a need for optical systems including optical amplifiers and optical switching devices that provide increased control over signal channels being switched, or routed, through the system.
As the need for high capacity WDM systems continues to grow, it will become increasingly necessary to provide all optical networks that eliminate the need for and expensive of electrical regeneration to perform signal routing and grooming in the networks. The development of multi-dimensional, all-optical networks will provide the cost and performance characteristics required to further the development of high capacity, more versatile, longer distance communication systems.